New donor-pi-acceptor (D-pi-A) type conjugated copolymers, poly[(4,8-bis((2-hexyldecyl)oxy)benzo[1,2-b:4,5-b'] dithiophene)-alt-(2,5-bis(4-octylthiophen-2-yl)thiazolo[5,4-d]thiazole )] (PBDT-tTz), and poly[(4,8-bis((2-hexyldecyl)oxy)benzo[1,2-b:4,5-b']dithiophene)-alt-(2,5-bis(6-octylthieno[3,2-b]thiophen-2-yl)thiazolo[5,4-d]thiazole)] (PBDT-ttTz) were synthesized and characterized with the aim of investigating their potential applicability to organic photovoltaic active materials. While copolymer PBDT-tTz showed a zigzagged non-linear structure by thiophene pi-bridges, PBDT-ttTz had a linear molecular structure with thieno[3,2-b]thiophene pi-bridges. The optical, electrochemical, morphological, and photovoltaic properties of PBDT-tTz and PBDT-ttTz were systematically investigated. Furthermore, bulk heterojunction photovoltaic devices were fabricated by using the synthesized polymers as p-type donors and [6,6]-phenyl-C-71-butyric acid methyl ester as an n-type acceptor. PBDT-ttTz showed a high power conversion efficiency (PCE) of 5.21% as a result of the extended conjugation arising from the thienothiophene pi-bridges and enhanced molecular ordering in the film state, while PBDT-tTz showed a relatively lower PCE of 2.92% under AM 1.5G illumination (100 mW/cm(2)). (C) 2017 Wiley Periodicals, Inc.